Method of eliminating discomforting flickering when viewing X-ray film in a light cabinet, and a flicker-eliminating unit for use in a light cabinet

ABSTRACT

A method of eliminating discomforting flickering when viewing X-ray films in a light cabinet using fluorescent lamps, and a flicker-eliminating circuit for use in the light cabinet.

TECHNICAL FIELD

Personnel working with X-ray film viewing cabinets often complain oftiredness and headaches, resulting from flickering light emitted by theflourescent strip lighting with which the X-ray pictures areilluminated. This is noticed most when examining underexposed parts ofX-ray film, where it is necessary to decrease the light strength inorder to obtain the requisite contrasting effect. In this case,flickering becomes troublesome after only a short time.

One serious, indirect disadvantage with flickering of the flourescenttubes, is that the X-ray technician or the doctor is often unable tomake a positive diagnosis on the basis of an existing X-ray picture, andhence it is necessary to take a new picture, with the patient beingsubjected to a further dose of X-rays as a result thereof.

OBJECT OF INVENTION

The present invention relates to a method of eliminating the flickeringof fluorescent tubes in X-ray cabinets with the aid of simple means,thereby eliminating the aforementioned serious disadvantages.

BRIEF SUMMARY OF INVENTION

According to one aspect the invention resides in a method of eliminatingdiscomforting flickering when examining X-ray film in a light cabinetwith fluorescent strip lighting supplied with mains voltage viathermionic transformers, for regulating the light intensity. The methodis mainly characterized by stabilizing and rectifying the mains voltage,and by energizing the fluorescent tubes subsequent to increasing thefrequency to at least twice the mains frequency, suitably to at least 10kHz and preferably at least 20 kHz. By increasing the frequency of thelighting supply voltage, flickering is reduced or eliminated, which isadvantageous, particularly when the light intensity of the lighting isreduced so as to enable underexposed parts of X-ray films to beexamined.

A clear and, in many cases, acceptable effect can be obtained byincreasing said frequency to approximately twice that of the mainsfrequency, i.e. to about 100-120 Hz. A fully satisfactory effect isnormally obtained, however, when the frequency reaches about 10 kHz. Thehigher the frequency, the lower the risk of the human ear discerninggenerated sound oscillations, which risk is considered to besubstantially non-existant at frequencies above 20 kHz. Because of this,and for other reasons, a frequency of this order of magnitude ispreferred. In addition hereto, a completely flicker-free light isobtained from fluorescent lighting at such frequencies, irrespective ofthe light intensity.

According to a preferred embodiment of the invention, the increase infrequency is effected by means of an oscillator which co-oscillates withone or more power drive transistors for supplying the primary side of anoutput transformer whose secondary side delivers a voltage whichcorresponds to the nominal mains voltage and which is applied to thefluorescent tubes via current-limiting chokes.

These cokes impede the current in accordance with current specificationsfor existing fluorescent tubes. Chokes intended for frequencies of from20-25 kHz are considerably smaller in size and cheaper in price than thepresent day conventional chokes for frequencies of 50 Hz.

The utilization of a pulse width modulated oscillator enables the lightintensity of the fluorescent tube to be readily regulated via a variableresistance.

According to a further embodiment of the invention, the mains voltage isstabilized prior to rectification via a transformer, by registering onthe secondary side of the transformer any deviation from the nominalmains voltage, and correcting said deviations by switching abidirectional thyristor arranged on the primary side of the transformer.In this way the mains voltage is stabilized reliably, which is necessaryif the subsequent rectification and increase in frequency is to lead tothe desired result.

According to a second aspect, the invention relates to aflicker-eliminating arrangement for a light cabinet having fluorescenttubes supplied with mains voltage via a filament transformer, forregulating the light intensity, the main features of said arrangementbeing set forth in the following claims.

An arrangement according to the invention can either be installed inX-ray film light cabinets during the manufacture of said cabinets, orcan be installed in existing cabinets, thereby enabling theaforementioned serious occupational hazards represented by theflickering of fluorescent tubes provided in said cabinets to beeffectively eliminated in a simple and inexpensive fashion.

BRIEF DESCRIPTION OF DRAWING FIGURE

An exemplary embodiment of the invention will now be described in moredetail with reference to the circuit diagram illustrated in the singleFIGURE of the drawing accompanying this specification.

DESCRIPTION OF PREFERRED EMBODIMENT

As shown in the drawing, the arrangement, which is intended to bemounted in an X-ray examining light cabinet, comprises five major parts,these being a voltage-stabilized mains unit, a filament transformer, anoscillator, an output transformer with power drive transistors, andfluorescent tubing belonging to the cabinet.

The stabilized mains unit is connected to the mains via a fuse F1, tostabilize the mains voltage via a transformer T1. The mains voltage isstabilized by means of bridge-coupled diodes V2, and is filtered bymeans of a capacitor C1, so as to obtain a smooth, non-pulsing d.c.voltage U_(c).

The loaded voltage is measured at X1, and any deviation from the ratedvoltage is corrected by switching a bidirectional thyristor ("triac")V1.

As a result of stabilizing and rectifying the mains voltage, the voltageof the fluorescent tube is not affected by any other load on the mainsnetwork, or by unreliable mains qualities in general resulting an impuremains voltage sine curve.

The filament transformer T2 is connected to the mains network via thefuse F1 and provides voltage to both electrodes of the fluorescent tube.As a rule this voltage is low and may, for example, be 4 volts. Thisenables the light to be controlled or regulated in a known manner, forexample by pulse width modulation.

The output transformer having the power drive transistors V3 and V4provide the fluorescent tubes with a.c. voltage, via chokes L1 and L2,having a frequency which is higher than about twice the frequency of themains voltage and which preferably reaches 20 kHz or more, said chokesbeing arranged to limit the current for present day types of fluorescenttubes in accordance with current specifications. The output transformerT3 transforms the voltage to a value corresponding to the nominal orrated mains voltage.

The oscillator which is shown as a pulse-width modulated oscillator inthe sole FIGURE switches the transistors V3 and V4 with given frequency.If this frequency reaches 20-25 kHz, frequencies audible to the humanear are eliminated, which is of particular advantage when there is nomechanical insulation.

As beforementioned, the light intensity of the fluorescent tubes can bevaried by pulse-width modulation, which is regulated via the variableresistance R1.

Normally, a conventional light cabinet includes the fuse F1, thetransformer T2 and one or more fluorescent tubes. Current is supplieddirectly from the mains network, via suitable chokes.

Thus, in accordance with a preferred embodiment of the invention othercomponents illustrated in the drawing are included in the componentarrangement which according to one aspect of the invention can be usedto convert a conventional light cabinet in which the tubes are likely toflicker into a light cabinet in which flickering will not occur.

INDUSTRIAL APPLICABILITY

The component arrangement can be readily installed. Thus, it is normallyonly necessary to sever four of the electrical conductors of theelectrical system of a conventional light cabinet and connect thereto acomponent arrangement or unit according to the invention, said unithaving connecting means which correspond in number to the number ofsevered conductors. No complicated reconstruction of the actual lightcabinet is required; neither is it necessary as a rule to change themanner in which the conductors are arranged and drawn.

It will be obvious to one of normal skill in this art that the frequencycan be increased in a different way to that described. Thus, thedescribed and illustrated embodiment is only to be considered an exampleof the invention, which can be modified in many ways within the scope ofthe following claims.

I claim:
 1. A method of eliminating discomforting flickering whenviewing an under-exposed portion of X-ray film in a light cabinet havingfluorescent tube means supplied with alternating current mains voltageof a given frequency via a filament transformer, and for regulating thelight intensity, said method comprising the steps of:stabilizing andrectifying the mains voltage to obtain a substantially smooth andnon-pulsing d.c. voltage; viewing said underexposed portion of saidX-ray film; generating a nominal voltage having a predeterminedfrequency determined by:setting a level of voltage utilizing said d.c.voltage to reduce flickering of said fluorescent tube means whileviewing said X-ray film; generating pulse-width signals wherein thepulse width and the magnitude thereof are determined by said level ofvoltage; transforming said pulse-width signals into said nominal voltagesuch that said predetermined frequency corresponds to said pulse widthof said pulse-width signals; choking said nominal voltage having saidpredetermined frequency to impede the current in accordance with thecurrent requirements for said fluorescent tube means, applying thechoked nominal voltage to said fluorescent tube means.
 2. A methodaccording to claim 1, wherein the step of generating pulse-width signalsis accomplished by the co-oscillation of power drive transistors independence upon said level of voltage.
 3. A method according to claim 1or claim 2, wherein the mains voltage is stabilized by registeringdeviations thereof and subsequently correcting any deviation byswitching
 4. A flicker-eliminating component arrangement for an X-rayviewing light cabinet having fluorescent tube means having an operatingcurrent supplied with alternating current mains voltage of a givenfrequency via a filament transformer, for regulating the light intensityand viewing an underexposed portion of an X-ray film, said arrangementcomprising:stabilizing means for stabilizing and filtering the mainsvoltage; means for rectifying the stabilized and filtered mains voltageto produce a substantially smooth d.c. voltage; means for viewing saidunderexposed portion of said X-ray film; circuit means for generating anominal voltage having a predetermined frequency including:means forsetting a level of voltage utilizing said d.c. voltage to reduceflickering of said fluorescent tube means while viewing said X-ray film;means for generating pulse-width signals wherein the pulse width and themagnitude thereof are determined by said level of voltage; transformermeans supplied with said pulse-width signals, said transformer meansgenerating said nominal voltage such that said predetermined frequencyof said nominal voltage corresponds to said pulse width of saidpulse-width signals; current-limiting means connected between saidcircuit means and said fluorescent tube means for limiting currentsupplied thereto.
 5. A component arrangement according to claim 4,wherein said circuit means comprises an oscillator arranged toco-oscillate, via power drive transistors, with the primary side of anoutput transformer, said power drive transistors generating pulse-widthsignals in accordance with said level of voltage supplied to saidoscillator, said transformer arranged to transform the voltage to saidnominal voltage, and the output side of said transformer being connectedto the current-limiting means.
 6. A component arrangement according toclaim 4 or claim 5, wherein the stabilizing means comprises means forregistering deviations from said nominal mains voltage on the secondaryside of an input transformer and means for correcting said deviations byswitching a bidirectional thyristor located on the primary side of theinput transformer.